Corn, Zea mays L., is one of the crops most widely grown in the United States, with over 60 million acres of corn planted annually (Andow and Hutchison (1998) “Bt-corn resistance management”. In Now or never: serious new plans to save a natural pest control, eds. Mellon and Rissler, eds., pp. 19-66, Union of Concerned Scientists, Cambridge, Mass.). Fall armyworm (FAW, Spodoptera frugiperda (J. E. Smith)) is one of the most important lepidopteran pests of corn in southern United States (Buntin (2008) Florida Entomol. 91:523-530), as well as Latin and South Americas. Damage by FAW involve leaf feeding, often observed in whorl stage plants, as well as ear feeding, causintg substantial yield losses. Insecticidal control to prevent ear damage in field corn is difficult and generally not cost effective. Transgenic corn expressing Bacillus thuringiensis (Bt) insecticidal toxins is an effective control technology against FAW offering great potential for reducing losses by this insect pest in field corn (Buntin et al. (2001) Florida Entomol. 84:37-42; Buntin et al. (2004) J. Econ. Entomol. 97:1603-1611). However, there is a concern that insects may rapidly develop resistance to the Bt expressed in plants in areas where continuous use and intensive selection pressure is applied (Mallet and Porter (1992) Proc. R. Soc. B 250:165-169; Chaufaux et al. (2001) J. Econ. Entomol. 94:1564-1570).
Insect resistance evolution has been well documented and is a serious problem in agricultural and livestock production, urban environments, and public health (Georghiou (1986) “The magnitude of resistance problem,” In Pesticide Resistance: strategies and tactics for management, Council, ed., pp. 14-44, National Academy Press, Washington, D.C.; Roush and McKenzie (1987) Annu. Rev. Entomol. 32:361-380, Roush and Tabashnik (1990) Pesticide resistance in arthropods, New York, N.Y., Chapman and Hall). Bt is a valuable source of insecticidal proteins for use in insect pest control either in conventional spray formulations or in transgenic crops (Roush (1994) Biocontrol Sci. Technol. 4:501-516; Ferré and J. Van Rie (2002) Annu. Rev. Entomol. 47:501-533). Nonetheless, the evolution of insect resistance in field populations is an important threat to this technology (Ferré and J. Van Rie (2002) Annu. Rev. Entomol. 47:501-533), especially with transgenic plants that express Bt toxins (Mallet and Porter (1992) Proc. R. Soc. B 250:165-169).
Maize hybrids containing event TC1507 express both Cry1F and PAT genes. The Cry1F protein confers resistance to key Lepidopteran pests of maize, such as European corn borer (Ostrinia nubilalis), southwestern corn borer (Diatraea grandiosella), FAW, and black cutworm (Agrotis ipsilon). The pat gene encodes the PAT protein to confer tolerance to the herbicidal active ingredient glufosinate-ammonium. Maize hybrids containing event TC1507 have been widely adopted in the United States since its commercialization in 1998. As part of the regulatory submission a mandated insect resistance management (IRM) plan was proposed to delay the rate of evolution of resistance. Currently, the preferred and most widely adopted strategy involves the use of plants expressing a high dose of the Bt toxin in conjunction with planting a refuge of a non-Bt crop for preservation of susceptible genes (International Life Sciences Institute. Health and Environmental Sciences Institute (1999) An evaluation of insect resistance management in Bt field corn: A science-based framework for risk assessment and risk management; Tabashnik et al. 2003. J. Econ. Entomol. 96:1031-1038). This approach was considered to be most feasible and realistic in terms of farming practices and in prolonging the use of Bt transgenic crops (Gould (1998) Annu. Rev. Entomol. 43:701-726). However, there still is a concern that insects may develop resistance to the Bt expressed in plants in areas where continuous use and intensive selection pressure is applied (Mallet and Porter (1992) Proc. R. Soc. B 250:165-169; Chaufaux et al. (2001) J. Econ. Entomol. 94:1564-1570).
FAW populations in Puerto Rico have been exposed to microbial Bt formulation used in conventional insecticides, and to transgenic plants containing event TC1507 over several years, both containing Bt Cry1 insecticidal proteins. Even though the Cry1F toxin is uniquely efficacious in controlling FAW when compared to other Cry1 toxins (Waquil et al. (2002) Revista Brasileira de Milho e Sorgo 1:1-11; Waquil et al. (2004) Revista Brasileira de Milho e Sorgo 3:161-171), repeated exposures to this toxin and the unique conditions of Puerto Rico (i.e., tropical island geography, reduced availability of alternative hosts due to drought conditions, continuous corn growth, and high population density with overlapping generations) collaborated for increased pest population selection pressure and therefore increased likelihood for evolution of resistance.